When \(c>0\) , \(O\)  is clearly a source when \(a<0\) 
and a sink when \(a>0\) .
Furthermore,
when \(c>0\)  and \(a=0\) ,
we can check that
\(O\)  is a stable fine focus of order 1 when \(b>0\) 
and an unstable fine focus of order 1 when \(b<0\)  according to [1]}.
Moreover, when \(c>0\)  and \(a=b=0\) ,
\(O\)  is a stable fine focus of order 2 according to Bautin bifurcation Theorem of [2]}.
\(O\)  is a Bautin point.
Thus, \(H_1\)  and \(H_2\)  are two Hopf bifurcation surfaces for \(c>0\) .
